The conversion of propylene oxide using base-catalyzed conditions to produce a mixture of monopropylene glycol methyl ethers (PM), dipropylene glycol methyl ether (DPM), tripropylene glycol methyl ethers (TPM) and heavier molecular weight polypropylene glycol methyl ethers is the current industry standard technology for commercial PM glycol ethers. The mixture of the mono-, di-, tri- and heavier product categories can be controlled by adjusting the methanol-to-propylene oxide feed mole ratio, recycling products back to the reactor for further propylene oxide addition, and adjusting the reactor temperature among other means.
The monopropylene glycol methyl ether family includes two isomers, 1-methoxy-2-propanol (PM2) and 2-methoxy-1-propanol (PM1). Using industry standard base-catalyzed technology, the PM2/PM1 ratio is ˜20/1. Reaction technology giving a selectivity >20/1 is preferable since PM1 is classified as a teratogen and can be present as a component in the commercial PM2 product at <0.5 wt %. In order to achieve this specification, costly distillation is used to separate these similar boiling materials (PM2 bp=118-119° C.; PM1 bp=130° C.).
Catalytically propoxylating PM1 to propoxylated adducts with very little reaction of PM2 can provide a mixture that is easily separated by simple distillation yet retains the highly desired PM2 product. Moreover, a catalyst system that selectively propoxylates methanol to monopropylene glycol methyl ether and at the same time further catalyzes the selective propoxylation of the undesired PM1 product to DPM can result in a highly selective process for producing PM2.
Although the primary hydroxyl group of PM1 is more acidic than the secondary hydroxyl group of PM2, significant propoxylation of both PM1 and PM2 occurs under base-catalyzed (e.g., NaOH or KOH) conditions.
Therefore, a process in which a Schiff base metal complex is used to selectively catalyze PM1 propoxylation to DPM in the presence of PM2 with very little reaction of PM2 would be desirable. Further, a process which yields very little PM1 by performing selective reaction of methanol with propylene oxide over a Schiff base metal complex catalyst followed by further selective reaction of the undesired PM1 product to DPM over the same Schiff base catalyst would also be desirable. Additionally, novel compositions which can be used as catalysts for the regio selective methanolysis of propylene oxide would also be desirable.